The Applicant has developed a range of Memjet® inkjet printers as described in, for example, WO2011/143700, WO2011/143699 and WO2009/089567, the contents of which are herein incorporated by reference. Memjet® printers employ a stationary pagewidth printhead in combination with a feed mechanism which feeds print media past the printhead in a single pass. Memjet® printers therefore provide much higher printing speeds than conventional scanning inkjet printers.
In order to minimize the amount of silicon, and therefore the cost of pagewidth printheads, the nozzle packing density in each silicon printhead IC needs to be high. A typical Memjet® printhead IC contains 6,400 nozzle devices, which translates to 70,400 nozzle devices in an A4 printhead containing 11 Memjet® printhead ICs.
This high density of nozzle devices poses a thermal management problem: the ejection energy per drop ejected must be low enough to operate in so-called ‘self-cooling’ mode—that is, the chip temperature equilibrates to a steady state temperature well below the boiling point of the ink via removal of heat by ejected ink droplets.
Conventional inkjet nozzle devices comprise resistive heater elements coated with a number of relatively thick protective layers. These protective layers are necessary to protect the heater element from the harsh environment inside inkjet nozzle chambers. Typically, heater elements are coated with a passivation layer (e.g. silicon dioxide) to protect the heater element from corrosion and a cavitation layer (e.g. tantalum) to protect the heater element from mechanical cavitation forces experienced when a bubble collapses onto the heater element. U.S. Pat. No. 6,739,619 describes a conventional inkjet nozzle device having passivation and cavitation layers.
However, multiple passivation and cavitation layers are incompatible with low-energy ‘self-cooling’ inkjet nozzle devices. The relatively thick protective layers absorb too much energy and require drive energies which are too high for efficient self-cooling operation.
To some extent, the requirement for a tantalum cavitation layer can be mitigated by ensuring the device vents bubbles through the nozzle aperture instead of the bubbles collapsing onto the heater element. Moreover, durable corrosion-resistant materials, such as titanium aluminium nitride (TiAlN), may be employed as heater materials. As described in U.S. Pat. No. 7,147,306, the contents of which are incorporated herein by reference, a naked TiAlN heater element may be employed in direct contact with ink, providing excellent thermal efficiency and no loss of energy into protective layers. TiAlN heater materials have the ability to form a self-passivating, native aluminium oxide coating. The oxide formation is self-limiting in the sense that it prevents further oxide formation and minimizes heater resistance rise. However, the protective oxide is susceptible to attack by other corrosive species present in inks e.g. hydroxyl ions, dyes etc.
Atomic layer deposition (ALD) is an attractive method for depositing relatively thin protective layers onto heater elements in inkjet nozzle devices in order to improve printhead lifetimes. Thin protective layers (e.g. less than 50 nm thick) have minimal effect on thermal efficiency, enabling low ejection energies and facilitating self-cooling operation.
US2004/0070649 describes deposition of a dielectric passivation layer and a metal cavitation layer onto a resistive heater element using an ALD process.
U.S. Pat. No. 8,025,367 describes an inkjet nozzle device comprising a titanium aluminide heater element having passivating oxide. The heater element is optionally coated with a protective layer of silicon oxide, silicon nitride or silicon carbide by conventional CVD.
U.S. Pat. No. 8,567,909 describes deposition of a laminated stack comprising alternating layers of hafnium oxide and tantalum oxide onto a TiN heater element (as described in U.S. Pat. No. 6,739,519) using an ALD process. According to the authors of U.S. Pat. No. 8,567,909, the laminated stack minimizes the effects of so-called pinhole defects through the thin protective layers. Pinhole defects in ALD layers potentially enable penetration of corrosive ions through to the heater element. By employing a stack of alternating materials, alignment of pinhole defects between layers is minimized and, therefore, this type of laminated structure minimizes corrosion. However, a drawback of employing a laminated stack of ALD layers is increased fabrication complexity.
It would be desirable to provide inkjet nozzle devices having improved lifetimes. It would be particularly desirable to provide a self-cooling inkjet nozzle device, which ejects at least one billion droplets over a lifetime of the device and has minimal fabrication complexity.